The present invention relates generally to automotive vehicle steering systems and, in particular, to a rack bushing used in a rack and pinion steering assembly.
A typical hydraulic rack and pinion power steering assembly used in a vehicle includes a rack that extends axially through a hydraulic chamber inside a hydraulic housing. The ends of the rack project axially outward from each end of the housing and are connected with steering linkages that connect to steerable vehicle wheels. A piston is fixed to the rack within the hydraulic chamber. Rotation of the vehicle steering wheel actuates a hydraulic valve and causes hydraulic fluid under pressure to act against the piston. The force exerted by the hydraulic fluid moves the piston within the housing, causing the rack to move axially. The axial movement of the rack moves the steering linkages that, in turn, cause the corresponding vehicle wheels to turn.
To prevent hydraulic fluid from flowing out of the housing, it is necessary to seal the ends of the housing through which the rack projects. It is also desirable to support the rack for axial movement within the housing without losing the integrity of the seal. A bushing and seal assembly is typically used for this purpose. It is also known to enclose the ends of the housing with bellows attached to the outside of the housing to prevent moisture, dirt or other contaminants from entering and interfering with the operation of the rack.
Machined or cast metal rack bushings that employ inner liners for bearing against the rack and outer rubber O-rings for sealing against the housing are known. In particular, these metal rack bushings are capable of withstanding the side loads induced on the bushing during vehicle operation. But such bushing assemblies can be more expensive than is desired and require these separate parts for the bushing assembly. Molded plastic rack bushings have also been employed. While these plastic bushings are more cost-effective than the metal bushings, they are less capable of accepting the high side loads that may be induced in the bushing during vehicle operation.
In addition, axially extending voids with generally rectangular shaped cross sections, also known as metal savers, have been included in the body of the rack bushing. While these voids are helpful in reducing the overall weight of the rack bushing assembly, the corners of these voids tend to create stress risers in the bushing.
Thus, it is desirable to have a rack bushing employed with a rack and pinion steering assembly that overcomes the drawbacks of the prior art.